Combustion is an exothermic oxidation reaction in which the heat evolved results from the formation of carbon oxygen bonds. For each carbonaceous material there is a specific carbon to oxygen ratio that ideally corresponds to complete or stoichiometric combustion. The terms "complete combustion" and "stoichiometric combustion" are used herein to refer to the conversion of a carbonaceous material to CO.sub.2 with no CO being produced. Complete combustion is often desirable because the production of heat is maximized and the production of pollutants (e.g., carbon monoxide) is eliminated. Complete or stoichiometric combustion of methane, acetylene, ethylene, ethane, propane, butane and benzene can be represented by the following equations: EQU CH.sub.4 +20.sub.2 =CO.sub.2 +2H.sub.2 O EQU C.sub.2 H.sub.2 +2.50.sub.2 =2CO.sub.2 +H.sub.2 O EQU C.sub.2 H.sub.4 +30.sub.2 =2CO.sub.2 +2H.sub.2 O EQU C.sub.2 H.sub.6 +3.50.sub.2 =2CO.sub.2 +3H.sub.2 O EQU C.sub.3 H.sub.8 +50.sub.2 =3CO.sub.2 +4H.sub.2 O EQU C.sub.4 H.sub.10 +6.50.sub.2 =4CO.sub.2 +5H.sub.2 O EQU C.sub.6 H.sub.6 +7.50.sub.2 =6CO.sub.2 +3H.sub.2 O
In the above equations, 2 moles of oxygen are required per mole of methane to achieve complete or stoichiometric combustion of methane; 2.5 moles of oxygen are needed per mole of acetylene to achieve complete or stoichiometric combustion of acetylene; 3 moles of oxygen per mole of ethylene are needed to achieve complete or stoichiometric combustion of acetylene; etc. Similar equations can be used to represent the complete or stoichiometric combustion of other carbonaceous materials. The amount of air required for a stoichiometric mixture for many carbonaceous fuels is provided in Perry, J. H., et al, Editors, "Chemical Engineer's Handbook", Fourth Edition (1963) at pp. 9-31 to 9-33.
In large-scale or commercial operations involving the combustion of carbonaceous fuels, it is usually not possible to obtain complete combustion with only a stoichiometric amount of oxygen or air. It is thus common practice to add excess oxygen or air (that is, oxygen or air in excess of stoichiometric amount required to provide complete combustion) to effect complete or substantially complete combustion. Excess oxygen or air is typically added to the combustion reaction until carbon monoxide in the product gases is eliminated or reduced to acceptable levels. The amount of excess oxygen or air required depends on many factors including the particular carbonaceous fuel being burned, the type of burner or furnace being used, etc. A gaseous fuel can be easily mixed with oxygen or air and thus is the easiest type of carbonaceous fuel with which to obtain complete combustion. Commercial burners are available which can operate with as little as up to about 10% excess air and still obtain complete combustion. Liquid fuels are less easily mixed with air and often require up to about 20% or more excess air. Solid fuels often require from up to about 50% or more excess air for economical combustion and usually still leave some unburned carbon in the ash residue. Adding excess oxygen or air has certain disadvantages. The excess oxygen or air decreases the efficiency of the combustion process by reducing its ultimate obtainable temperature and by increasing the size of the equipment necessary to convert all of the carbonaceous fuel to carbon dioxide. The use of oxidation catalysts to enhance combustion and thereby eliminate the requirement for excess oxygen or air has been suggested, but to date no oxidation catalyst has been developed that is entirely satisfactory.
The concept of combusting or incinerating organic waste originated in England about 100 years ago. The first U.S. incinerator was built on Governor's Island, N.Y., in 1885, and the first municipal incinerator was one of 27 metric tons per day capacity constructed at Allegheny City, Pa., in the same year. By 1921 more than 200 municipal incinerators were in operation. These early incinerators were little better than an enclosed bonfire. Small, individual, backyard incinerators were a popular means of disposal of waste in the 1930-1940's in communities without waste-collection facilities. They were, however, sources of air pollution and are now generally prohibited. Today it is possible to build large, advanced central incineration plants which are virtually nuisance-free and environmentally acceptable.
Due to the cost of energy consumption, combustion is not the cheapest method for the disposal of organic waste. Landfill is generally cheaper, but the growing shortage of disposal sites near population centers, the increasing cost of transportation, and the growing reluctance of smaller communities and rural areas to accept waste from other localities has significantly reduced the viability of this technique as a future disposal method. Composting and biodegradation of organic wastes has not been successful on a large scale.
The disposal of solid waste and/or hazardous waste, including petroleum waste and refinery sludge is particularly troublesome. The term "solid waste" refers to any garbage, sludge or other solid organic waste material. The term "hazardous waste" refers to solid waste or combinations of solid waste which are "listed" by the Environment Protection Agency (EPA) as hazardous, or which exhibit ignitability, corrosivity or reactivity, or are considered toxic pursuant to relevant governmental rules or regulations. The term "petroleum waste" refers to any waste material containing petroleum or hydrocarbon oil; petroleum waste can have particulate solids and/or water intermixed with it. The term "refinery sludge" refers to sludges generated in petroleum refinery operations that contain petroleum or hydrocarbon oils; these sludges can contain particulate solids as well as water and the hydrocarbon oils in these sludges usually contain heavy residual hydrocarbons including asphaltenes. Historically, these waste materials have been economically disposed of by land-filling and land-farming techniques. However, land disposal is now regulated by the Resource Conservation and Recovery Act (RCRA) and the Hazardous and Solid Waste Amendments of 1984 (RITA), and has consequently become more difficult and expensive.
Combustion or incineration appears to be an environmentally acceptable means for the disposal of organic waste, but the cost of this technique has limited its application. Catalytic combustion or incineration has been suggested for the purpose of reducing the operating temperature and thus the cost of such combustion or incineration. Catalysts that have been suggested include noble metals (e.g., platinum, palladium) dispersed on the surface of a catalyst support (e.g., silica honeycomb or screen of nichrome wire). These catalysts have, however, been found to be costly and subject to poisoning or blanketing which reduces their activity.
A major source of methane is natural gas which typically contains about 40-95% methane depending on the particular source. Other constituents include about 10% of ethane with the balance being made up of CO.sub.2 and smaller amounts of propane, the butanes, the pentanes, nitrogen, etc. Primary sources for natural gas are the porous reservoirs generally associated with crude oil reserves. From these sources come most of the natural gas used for heating purposes. Quantities of natural gas are also known to be present in coal deposits and are by-products of crude oil refinery processes and bacterial decomposition of organic matter.
Prior to commercial use, natural gas must be processed to remove water vapor, condensible hydrocarbons and inert or poisonous constituents. Condensible hydrocarbons are generally removed by cooling natural gas to a low temperature and then washing the natural gas with a cold hydrocarbon liquid to absorb the condensible hydrocarbons. The condensible hydrocarbons are typically ethane and heavier hydrocarbons. This gas processing can occur at the wellhead or at a central processing station. Processed natural gas typically comprises a major amount of methane, and minor amounts of ethane, propane, the butanes, the pentanes, carbon dioxide and nitrogen. Generally, processed natural gas comprises from about 70% to more than about 95% by volume of methane.
Most processed natural gas is distributed through extensive pipeline distribution networks. As natural gas reserves in close proximity to gas usage decrease, new sources that are more distant require additional transportation. Many of these distant sources are not, however, amenable to transport by pipeline. For example, sources that are located in areas requiring economically unfeasible pipeline networks or in areas requiring transport across large bodies of water are not amenable to transport by pipeline. This problem has been addressed in several ways.
One approach has been to build a production facility at the site of the natural gas deposit to manufacture one specific product. This approach is limited as the natural gas can be used only for one product, preempting other feasible uses.
Another approach has been to liquefy the natural gas using cryogenic techniques and transport the liquid natural gas in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume occupied in the gaseous state by such cryogenic processing, and with proper procedures, safely stored or transported. These processes, which involve liquefying natural gas to a temperature of about -162.degree. C., transporting the gas, and revaporizing it are complex and energy intensive.
Still another approach involves the use of pyrolysis to convert the natural gas to higher molecular weight hydrocarbons (e.g., substantially liquid hydrocarbons) that can be easily handled and transported. Low-temperature pyrolysis (e.g., to 250.degree. C. and 500.degree. C.) is described in U.S. Pat. Nos.4,433,192; 4,497,970; and 4,513,164. The processes described in these patents utilize heterogeneous systems and solid acid catalysts. In addition to the solid acid catalysts, the reaction mixtures described in the 4,97,970 and 4,519,109 patents include oxidizing agents. Among the oxidizing agents disclosed are air, O.sub.2 -O.sub.3 mixtures, S, Se, SO.sub.3, N.sub.2 O, NO, NO.sub.3, F., etc. High-temperature pyrolysis (e.g., above about 1200.degree. C.) has been suggested. These high-temperature processes are, however, energy intensive and have thus far not been developed to the point where high yields are obtained even with the use of catalysts. Some catalysts that are useful in these processes (e.g., chlorine) are corrosive under such operating conditions.
Autothermal pyrolysis has been suggested as a means for overcoming the high energy requirements for high-temperature pyrolysis. In autothermal pyrolysis an oxidant (e.g., oxygen or air) is co-fed with the natural gas to the reactor. The reaction is substoichiometric in oxygen (that is, not enough oxygen is fed to completely consume, i.e., convert to CO.sub.x, all of the hydrocarbon in the feed mixture). The hydrocarbon not burned is available for pyrolysis reactions. In noncatalyzed autothermal pyrolysis processes, the major oxygen containing product is usually carbon monoxide and the major higher order hydrocarbon product is usually acetylene. In catalyzed autothermal pyrolysis, the catalyst provides a more efficient use of the oxygen and the major oxygen-containing product is carbon dioxide. The production of carbon dioxide provides a greater yield of heat than the production of carbon monoxide, and thus catalyzed autothermal pyrolysis processes are more efficient than non-catalyzed processes.
U.S. Pat. Nos. 3,926,854 and 3,947,380 disclose ceramic mixed oxide, non-stoichiometric electrically neutraI rare-earth-type catalysts containing rare-earth-type elements and elements of the first transition metal series and optionally alkaline earth metals. These catalysts have the following formula: EQU X.sub.n J.sub.( 1-n)ZO.sub.( 3.+-.m)
wherein: X is an alkaline earth metal or mixture thereof; J is a rare-earth-type element or mixture thereof; Z is a metal of the first transition series or a mixture thereof, at least 0.01% of said metal having an oxidation state other than +3; m is a number having a value of between zero and about 0.11; and n is a number having a value from zero to about 0.51. These patents indicated that these catalysts can be used to catalytically oxidize low molecular weight inorganic compounds and elements, such as ammonia, carbon monoxide, hydrogen, sulfur dioxide, and hydrogen sulfide, with oxygen, or carbon monoxide with water, sulfur dioxide or nitric oxide. The catalyst can also be employed in the catalytic removal of carbon monoxide, hydrocarbons and nitric oxides from the exhaust gases of generating or heating plants and automobiles burning fossil fuels.
U.S. Pat. Nos. 3,885,020; 3,976,599; 4,076,486; 4,124,689; and 4,124,697 disclose ceramic mixed oxide, non-stoichiometric electrically neutral, rare-earth-type catalysts containing rare-earth-type elements, elements of the first transition metal series and zirconium, tin or thorium and optionally alkaline earth metals. These catalysts have the following formula: EQU W.sub.k X.sub.n J.sub.(l-k-n) ZO.sub.( 3.+-.m)
wherein: W is zirconium, tin or thorium or mixture thereof; X is an alkaline earth metal or mixture thereof; J is a rare-earth-type element or mixture thereof; Z is a metal of the first transition series or a mixture thereof, at least 0.01% of said metal having an oxidation state other than +3; k is a number having a value of between zero and about 0.1; m is a number having a value of from zero to about 0.26, provided m has a value other than zero when n has a value of zero; and n is a number having a value from 0 to about 0.51 provided when n has a value of zero, k has a value of between zero and about 0.05. These patents indicate that these catalysts can be used to catalytically oxidize organic compounds to various states of oxidation, ammonia, carbon monoxide, hydrogen, sulfur dioxide, and hydrogen sulfide, with oxygen, or carbon monoxide with water, sulfur dioxide or nitric oxide. The catalyst can also be employed in the catalytic removal of carbon monoxide, hydrocarbons, nitric oxides and sulfur dioxide from the exhaust gases of generating or heatinq plants and automobiles burning fossil fuels. In addition, these catalysts can be employed to produce hydrogen cyanide from methane, ammonia and oxygen.